linux-kernel - Re: [PATCH] KVM: Fix assigned device MSI-X entry setting leak

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Date:	Mon, 30 Jan 2012 11:35:25 -0700
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: Fix assigned device MSI-X entry setting leak

On Mon, 2012-01-30 at 19:24 +0100, Jan Kiszka wrote:
> On 2012-01-30 19:06, Alex Williamson wrote:
> > We need to prioritize our matching when setting MSI-X vector
> > entries.  Unused entries should only be used if we don't find
> > an exact match or else we risk duplicating entries.  This was
> > causing an ENOSPC return when trying to mask and unmask MSI-X
> > vectors based on guest MSI-X table updates.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > 
> > ---
> > 
> > The faulting sequence went something like:
> > 
> > Start:
> > [0] entry 0, vector A
> > [1] entry 1, vector B
> > [2] entry 2, vector C
> > 
> > Set entry 1 to 0:
> > [0] entry 0, vector A
> > [1] entry 1->1, vector B->0
> > [2] entry 2, vector C
> > 
> > Set entry 2 to 0:
> > [0] entry 0, vector A
> > [1] entry 1->2, vector 0->0 <- incorrectly matched
> > [2] entry 2, vector C
> > 
> > Set entry 2 to C:
> > [0] entry 0, vector A
> > [1] entry 2->2, vector 0->C <- incorrectly matched again
> > [2] entry 2, vector C
> > 
> > Set entry 1 to B:
> > [0] entry 0, vector A
> > [1] entry 2, vector C
> > [2] entry 2, vector C
> > -ENOSPC
> > 
> > Userspace will need to test for this bug (or do we want to set
> > a feature flag?).
> 
> The latter is much simpler. We did it before.

Yeah, the test is little cumbersome, I'll send a v2 with a feature flag.

> > 
> >  virt/kvm/assigned-dev.c |   21 +++++++++++++++------
> >  1 files changed, 15 insertions(+), 6 deletions(-)
> > 
> > diff --git a/virt/kvm/assigned-dev.c b/virt/kvm/assigned-dev.c
> > index 758e3b3..798a090 100644
> > --- a/virt/kvm/assigned-dev.c
> > +++ b/virt/kvm/assigned-dev.c
> > @@ -728,7 +728,7 @@ msix_nr_out:
> >  static int kvm_vm_ioctl_set_msix_entry(struct kvm *kvm,
> >  				       struct kvm_assigned_msix_entry *entry)
> >  {
> > -	int r = 0, i;
> > +	int r = 0, i, unused = -1;
> >  	struct kvm_assigned_dev_kernel *adev;
> >  
> >  	mutex_lock(&kvm->lock);
> > @@ -741,17 +741,26 @@ static int kvm_vm_ioctl_set_msix_entry(struct kvm *kvm,
> >  		goto msix_entry_out;
> >  	}
> >  
> > -	for (i = 0; i < adev->entries_nr; i++)
> > -		if (adev->guest_msix_entries[i].vector == 0 ||
> > -		    adev->guest_msix_entries[i].entry == entry->entry) {
> > +	for (i = 0; i < adev->entries_nr; i++) {
> > +		if (unused < 0 && !adev->guest_msix_entries[i].vector)
> > +			unused = i;
> > +
> > +		if (adev->guest_msix_entries[i].entry == entry->entry) {
> >  			adev->guest_msix_entries[i].entry = entry->entry;
> >  			adev->guest_msix_entries[i].vector = entry->gsi;
> >  			adev->host_msix_entries[i].entry = entry->entry;
> >  			break;
> >  		}
> > +	}
> > +
> >  	if (i == adev->entries_nr) {
> > -		r = -ENOSPC;
> > -		goto msix_entry_out;
> > +		if (unused < 0) {
> > +			r = -ENOSPC;
> > +			goto msix_entry_out;
> > +		}
> > +		adev->guest_msix_entries[unused].entry = entry->entry;
> > +		adev->guest_msix_entries[unused].vector = entry->gsi;
> > +		adev->host_msix_entries[unused].entry = entry->entry;
> >  	}
> >  
> >  msix_entry_out:
> > 
> 
> Looks good. Does it fix the masking problems?

It appears to, a 4-way rhel6 guest w/ assigned igb happily shifts it's
smp affinity around with irqbalance while running netperf in a loop.
Thanks,

Alex

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